13. Why should a rocket’s CM be located noseward of its CP?

  • A “good rocket” is one that always rights itself by having its nose pivot into the wind.
  • The CM is the “pivot point” of any free-flying object. If a rocket rotates, it will rotate around its center of mass.
  • A rocket’sCPcan be thought of as the focal point of the pressure applied by the wind that is pushing against the rocket. It can also be thought of as the average location of all the pushing that the wind is doing. The force of the wind is proportional to the surface area against which it is pushing, so the CP is always situated toward the “big end” of the rocket.
  • If this focal point of the wind’s pushing (the CP) is situated tailward from the pivot point (the CM), then the rocket’s tail will be pushed/pivoted downwind. On the other hand, if the center of pressure is noseward from the center of mass, the rocket’s nose will be pushed/pivoted downwind, and the rocket will fly tail-first. In a third configuration, if the CP and the CM are in exactly the same location, the rocket will not pivot at all.
  • To summarize, the CM is the rocket’s pivot point. If the CP is tailward from the CM, then the wind will focus its pressure on the tail end of the rocket, pivoting that end backward and pivoting the nose end forward.

14. Explain why a well-designed weather vane always points into the wind?

A good weather vane has a pointer that points into the wind (e.g. it points north when the wind comes from the North). A weather vane pivots around a stick. In order for the weather vane to work, its CP should be on the opposite side of the stick from the pointer end of the weather vane. In a wind, the side of the vane with the CP will be pushed harder than the pointer end, tilting it downwind and tilting the pointer end upwind.

15.[there was no #15] Two different ways to make a flying object stable, and why they work:

  1. Add mass to the nose without changing the object’s surface area distribution. This shifts the CM noseward, so that it is located noseward of the CP.
  2. Add surface area to the tail of the object without changing the object’s mass distribution. This shifts the CP tailward, so that it is tailward of the CM.